A major goal of instruction is for students to develop competence with numbers. This study attempts to determine the effect of instructional materials that develop number concepts and connect these concepts to mental computation procedures. The study used 16 classes of eighth-grade students and a pretest-posttest design with two treatment groups. An experimental group used a set of author-written instructional materials as a supplement to the regular curriculum. Instructions to teachers stressed that student discussion of invented procedures was an important component of the instructional process and alternative methods of computation were to be encouraged. Results indicated that students showed a tendency to use standard procedures when whole numbers were involved, but when the problems involved decimals, fractions, and percents, students used methods which indicated a change away from the use of standard procedures to non-standard procedures. Significant gains were made in mental computation achievement. (MKR)